The invention is directed to a method for automatically setting illumination intensity for positional recognition devices and quality control devices during automatic placement of components on a printed circuit board or ceramic substrate.
When automatically equipping printed circuit boards or ceramic substrates with components, particularly surface mounted device (SMD) components, the position of the components relative to the placement position on the printed circuit board is identified before mounting the components using means for positional recognition. All articles capable of being equipped, for example shielding plates as well, are covered below by the term components. Optical system, comprised of a camera (e.g., a CCD camera), an illumination means are generally utilized as positional recognition devices. Visual inspection of the components ensues in analogously constructed devices for quality control. Illumination for these devices is selected such that useful structures (generally the terminals of the components) are shown with extremely high contrast, whereas unwanted structures (for example the body of the components) are suppressed.
Methods are known for the above purposes wherein the components are eliminated from different angles with different intensities with the assistance of illumination devices that are subdivided into a plurality of light sources each having a respectively separately controllable intensity.
JP 8-152311 (A), for example, discloses the illumination of soldered connections from different directions in order to inspect the quality of the soldered connections. The grayscale values identified in the individual illuminations are summed in an image evaluation unit. However, no teaching is given the document as to how illumination can be realized in an automatically adjustable way.
WO 96/21343, as another example, discloses a method wherein the components are not uniformly illuminated from all angles with the assistance of segmented ring illumination. Rather, no light is incident onto the structure from specific angles. Imagings of unwanted structures that may particularly emerge from these specific angles are thus suppressed.
A disadvantage of the known methods using a plurality of light sources is that an optimum illumination for the component must be manually set by an operator for each new component. To that end, the intensities of the individual light sources must be regulated by the operator until, under visual supervision, he has achieved an illumination adequate for the positional recognition or quality control. Hence, intensity setting in these known methods is user-specific and not reproducible, as would be required for equipping numerous circuit boards with large numbers of components.
U.S. Pat. No. 5,454,049 discloses a method for automatically setting illumination light source, wherein grayscale values registered with a camera and stored in an image evaluation unit are modified by an intensity control means such that the existing grayscale range of the image evaluation unit is optimized. However, this method is not suitable for illumination with a plurality of light sources, since the influence of the intensities of the individual light sources is not taken into consideration.
The present invention featured a reproducible, automatic method with which the intensity setting of the illumination is assured without fault in other-words at least two light sources used for positional recognition and quality control during equipping of a printed circuit board or substrate with components.
In order to achieve these features, the present invention includes a method for automatically setting illumination intensity for positional recognition devices and quality control devices used in automatic devices for placing components. Each of the positional recognition and quality control devices includes at least two or more light sources controlled by respective intensity control devices, a camera for registering an image of a component to be placed and an image evaluation unit connected to the camera that rasters the image of the component into picture elements in the form of grayscale values and registers at least a portion of the registered image. Further, picture elements of the image are divided into elements representing useful structures of the component and unwanted structures of the component. These respective structure elements are then allocated within the image evaluation unit to represent useful and unwanted structures.
The method includes these steps of successively eliminating the component according to varied illumination intensities by each respective light source. The image;of the component is then registered with the camera for each successive illumination. Grayscale values of the image registered by the camera are stored as grayscaled value parts in the image evaluation unit and respectively allocated as either representing useful structures or unwanted structures of the component, wherein the grayscale value ranges of the useful and unwanted structures are identified dependent on the illumination intensity of the individual light sources. The stored grayscale value parts are then summed in the evaluation unit for all of the light sources according to both the useful and the unwanted structures of the component, respectively. The respective grayscale values sums of the unwanted structures are then compared to the grayscale values of the useful structures.
Determination is then made whether the useful structures can be distinguished from the unwanted structures by identifying whether the difference between the grayscale:value sums of the unwanted and useful structures yields a substantially large difference. When at least one light source yields a substantial large difference between the grayscale value sums of the unwanted and the useful, structures, a desired maximum difference value is established. If the difference between the grayscale value sums does not yield a substantially large difference, a desired minimum difference value is-established. The difference values, whether maximum or minimum, are established utilizing a linear optimization. Finally, the illumination and intensities of the light sources are set at an optimum intensity based-on either the maximum difference value or the minimum difference value.
Compared to the known methods, the inventive method offers the advantage of achieving an automatic illumination setting that sequences independent of a user and in an automated fashion, (i.e., without involved, time-consuming trials) assuring a faultless illumination of a component in devices for positional recognition and quality control within an automatic equipping unit.
Additional advantages and novel features of the invention will be set forth in part in the description that follows and, in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The advantages of the present invention may be realized and attained by means of instrumentalities and combinations particularly pointed out in the appended claims.